"We had intensive and useful discussions," Iran's lead negotiator, deputy foreign minister Abbas Araqchi, told state television after a two-hour meeting in Geneva on Wednesday.
"All issues were discussed. We'll have a new meeting in a little under a month."
Araqchi held two days of talks with a US delegation headed by Acting Deputy Secretary of State Wendy Sherman ahead of the plenary session with the six powers.
The powers, which also include Britain, China, France, Germany and Russia, held formal talks with the Iranians in Vienna last month.
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They failed to meet a November 24 deadline for a comprehensive deal on reining in Iran's nuclear programme in exchange for an easing of crippling Western sanctions.
All parties agreed to give themselves seven more months -- until June 30 -- to strike a deal, although they said they hoped to have the broad outlines hammered out by March.
A final agreement is aimed at ensuring Tehran will never develop nuclear weapons under cover of its civilian activities.
Iran denies that it is seeking the bomb and insists its nuclear programme is for peaceful purposes only.